1. Field of the Invention
The present invention relates to a charged particle beam reflector which can correct chromatic aberration and spherical aberration of a lens system, and to an electron microscope incorporating the charged particle beam reflector.
2. Description of the Related Art
Generally, an electron beam used in an electron microscope has a wavelength of 0.024 Å when the acceleration voltage is 200 kV, for example. It is theoretically possible to increase a resolution of an electron microscope to the order of the wavelength of the electron beam; however, in practice a resolution of the electron microscope is 1.9 Å at most owing to chromatic aberration and spherical aberration of a lens system.
It is known that the chromatic aberration and spherical aberration of the lens system can be corrected by use of an electrostatic mirror. Japanese Laid-open Patent Application Publication No. Hei 5-205687 discloses an electron microscope including an electrostatic mirror to correct the chromatic aberration and spherical aberration of the lens system. Such an electron microscope comprises a beam deflector as a part of the imaging system and the electrostatic mirror to deflect an electron beam at 90 degrees by the beam deflector, reflect the deflected electron beam by the electrostatic mirror, and deflect the reflected electron beam at 90 degrees again by the beam deflector. In such a manner, it returns the electron beam to the original optical axis.
However, there is a problem in such an electron microscope that since the electron beam is deflected at 90 degrees by the beam deflector, electric and magnetic fields of the beam are both not rotationally symmetric, and the fringing effect of both fields is likely to cause an asymmetric high-order aberration.
Particularly, the high-order aberration occurring in reality and that theoretically obtained do not always coincide with each other, so that it is hard to suppress the high-order aberration and improve the resolution of the electron microscope to the order of the wavelength of the electron.